Crankshaft is supposed to be the heart of engine. Crankshaft converts reciprocating motion of piston into rotational motion. Crankshaft has complex geometry and it also experiences large number of load cycles during its service life. Therefore fatigue performance and durability are key considerations in crankshaft design and performance. During its service life, a crankshaft operates under high forces resulting from fuel combustion. The combustion and mass Inertia forces acting on the crankshaft cause two types of fluctuating loadings on the crankshaft structure i.e. torsional load and bending load. Hence a crankshaft requires high torsional and bending fatigue strength.
The designers of modern internal combustion engines are facing the challenge of reducing the environmental pollution to meet strict pollution control regulations all over the world. Environmental pollution can be reduced through improving engine efficiency. The crankshaft design is crucial in deciding the engine efficiency along with higher strength to weight ratio. Environmental compliance norms (for example, Euro norms) are becoming more and more stringent leading to tighter engine designs as a consequence of which the pressures to which engine cylinder are designed is much higher than a few years ago. To withstand this increase in pressure, crankshaft demands higher torsional and bending fatigue strength.
There is a constant search for high strength materials to cope with these demanding situations. One such high strength material category, the micro-alloy (MA) forging steel, has been finding increasing usage for gasoline and diesel engine crankshafts. Micro-alloy steel, also termed as micro-alloyed steel, is a type of steel that contains small amounts of alloying elements (0.05 to 0.15%). Standard alloying elements include: Niobium, Vanadium, Titanium, Molybdenum, Zirconium, Boron, and rare-earth metals. They are used to refine the grain microstructure and/or facilitate precipitation hardening.
The performance and cost of these steels lies between carbon steel and low alloy steel. Yield strength is between 500 and 750 MPa (73,000 and 109,000 psi) without heat treatment. Fatigue life and wear resistance are superior to those for similar but heat treated steels. The known disadvantages of the micro-alloy steels are that their ductility and toughness are not as good as quenched and tempered (Q and T) steels.
As a part of their forming process, the Micro-alloy steels must also be heated hot enough for the all of the alloys to be in solution. After forming, the material must be quickly cooled to 540 to 600° C. (1,004 to 1,112° F.) for grain refinement and at the same time cooling should be slow enough to ensure complete precipitation strengthening. The success of micro alloyed steels is due to strengthening mechanisms, specifically grain refinement and precipitation hardening.
Most new crankshaft applications specify micro-alloy steel, and many current applications are changing over from cast iron, or forged and heat treated plain carbon or alloy steels, to as-forged micro-alloy steels. Micro-alloy (MA) or high strength low alloy (HSLA) steel is an important development in the steel production and is used in every major steel market in various parts of world and the same has played an important role in expansion of industries such as oil and gas extraction, construction and transportation.
A variant of the MA steels, the Vanadium micro-alloyed steels tend to be coarser grained than the equivalent grade C38+N2 steels which do not contain Vanadium as shown in FIG. 1. It is therefore important to control the grain size using an optimum cooling rate from the forge temperature to refine grain size using a simple heat treatment which does not result in much distortion. Grain refinement process is one such treatment.
During forging process billets are heated up to 1280° C. It is observed from FIG. 1 that at the forging temperature the average grain diameter is more than 350 μm as represented by an equivalent ASTM grain size number of 0-1. Subsequently recrystallization takes place and new grains are formed. These grains are coarser (ASTM 3-5). It is known that with increase in grain size the endurance limit decreases. So it is essential to maintain fine grain size for better fatigue strength. In the case of a fine grained structure, the dislocation movement is restricted due to the increased amount of grain surface area whereby increased strength is achieved. Hence steel with fine grain size results into higher fatigue strength.
FIG. 1a shows a conventional micro-alloyed steel crankshaft manufacturing process deploys a forging operation followed by controlled cooling. The entire process comprises the steps of billet forming, forging followed by controlled cooling, checking the product for hardness, forming the final part through machining. Grain size obtained with this process is in the range of ASTM 3-5. Components such as crankshafts manufactured with this process are subsequently machined, induction-hardened, ground and tested for fatigue strength. Observed torsion fatigue strength of such crankshafts is between 95 to 100 MPa, and the bending fatigue strength is 3649 micro strain (839 MPa).